JPH0925507A - Method for refining molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently produce a high clean aluminum killed extra-low carbon steel by adding CaO during tapping molten steel roughly decarburized steel to lower the m.p. of slag, further executing an Al deoxidation after executing a vacuum decarburization and successively, executing a stirring refining. SOLUTION: The molten steel roughly decarburized steel in a refining furnace is tapped into a ladle and the molten steel in this ladle is vacuum-decarburized and refined in RH degassing equipment. At this time, CaO is added during tapping to lower the m.p. of slag and successively, the Al deoxidation is executed after vacuum deoxidizing in the condition of undeoxidation. The composition of ladle slag during modifying and after modifying the slag is desirable to be made into 5±0.2 basicity CaO/SiO2 and 1.5±0.1 CaO/Al2 O3 by charging the CaO. Thereafter, the molten steel is sufficiently stirred with the slag in simple refining equipment capable of the gas stirring and an alloy addition, and refined. By this method, the adjustment of components in the molten steel is executed, and also iron oxide in the slag is reduced to execute the slag modification.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the out-of-pile refining of molten steel, and in particular, the refining of molten steel for improving the cleanliness of aluminum-killed ultra-low carbon steel by rapidly and economically modifying the slag mixed in the refining vessel. Regarding the method.

[0002]

2. Description of the Related Art In the production of aluminum-killed ultra-low carbon steel, it is natural that the carbon content is in the ultra-low carbon range of several tens of ppm or less, but it is highly clean due to bending during use of the steel sheet. Sex is required.

The relationship between the cleanliness of molten steel and the degree of oxidation of slag has heretofore been clarified, and the total amount of iron oxide in molten steel (hereinafter referred to as (T.Fe)) decreases as the total iron oxide content of the slag (hereinafter referred to as (T.Fe)) decreases. Hereinafter, (TO) will be reduced. Therefore,
To improve the cleanliness of molten steel, the slag (T.Fe)
So-called slag reforming is carried out as an operation for reducing the above. Conventionally, various refining methods shown below have been proposed.

In Japanese Patent Laid-Open No. 59-70710, when the molten steel discharged from a steelmaking furnace to a ladle is secondarily smelted in a vacuum,
In order to add almost the required amount of deoxidizer as a product at the time of tapping steel from the steelmaking furnace, and to add a flux for promoting coalescence floating of the deoxidized product, or to further modify the oxidizing slag after that. A method (first method) for producing highly clean steel in which a slag reducing agent is added together is disclosed.

Further, in Japanese Unexamined Patent Publication No. 6-256836, after the vacuum decarburization treatment in the RH degassing furnace is completed, the sucking of the molten steel into the vacuum chamber is temporarily stopped, and the slag in the ladle is made of aluminum or the like. The method (second method) of adding the reducing agent is disclosed.

Further, in Japanese Patent Laid-Open No. 3-294414, the oxidizing slag is irradiated from above with an inert gas plasma in the presence of a reducing agent for the oxidizing slag floating on the molten steel in the refining vessel. When reducing and reforming, the basicity of the oxidizing slag is adjusted to 2 or more, and aluminum dissolved in molten steel is used as a reducing agent to convert the oxidizing slag to the final target component concentration of molten steel or higher. A method (third method) in which an amount sufficient for reduction is contained in advance or during refining, irradiation with an inert gas plasma and gas stirring of molten steel are disclosed.

[0007]

However, all of these conventional techniques have various problems. For example, in the first method, firstly, when the addition of the slag reducing agent is insufficient, the reduction of (T.Fe) ends up insufficiently, and secondly, the addition of the slag reducing agent increases the melting point of the slag. Since the slag solidifies, the Al ₂ O ₃ absorption capacity of the slag decreases, and the molten steel cleanliness decreases. Thirdly, the slag reducing agent added from above the ladle reacts with oxygen in the molten steel to produce a secondary vacuum. Oxygen in molten steel for decarburization is insufficient at the start of refining, which causes extension of decarburization time and decarburization failure. In particular, excessive addition of a reducing agent may make it impossible to decarburize.

In the second method, although the slag reforming is performed after the vacuum decarburizing treatment, the RH degassing treatment must be temporarily interrupted, and the recompression process, the ladle lowering process, and the slag reforming process are required. It is necessary to go through at least 5 steps of ladle raising process and depressurizing process, and productivity cannot be said to be high. Further, after the RH decarburization treatment is temporarily interrupted, the slag reducing agent is introduced from above the ladle, so that the slag reducing agent may be lost due to contact with the outside air.

Further, in the third method, although slag reforming is performed by using a deoxidizing element such as aluminum present in molten steel, an inert gas plasma must be used to dissolve the slag, Not only the equipment and energy costs increase, but also the melting temperature of the slag increases, so that the melting loss of the ladle refractory becomes significant and the refractory cost increases.

The object of the present invention is to reduce (T.Fe) of ladle slag in the secondary refining process of molten steel discharged from a converter and to increase Al _{2 of} the ladle slag.
(EN) A refining method for molten steel capable of improving O ₃ absorption capacity and more effectively producing highly clean aluminum killed ultra low carbon steel.

[0011]

A method for refining molten steel according to the present invention is such that molten steel that has been roughly decarburized in a refining furnace is tapped into a ladle, and then the molten steel in the ladle is vacuum decarburized by an RH degassing facility. In the method for refining molten steel, the melting point of slag is lowered by adding burnt lime to the tapped steel, then the undeoxidized molten steel is vacuum decarburized by the RH degassing equipment, and aluminum deoxidation is further performed. It is characterized in that the molten steel is refined in another simple refining equipment having a molten steel stirring function and an alloy addition function so that the slag and the molten steel are sufficiently stirred, and the composition adjustment of the molten steel and the slag reforming are simultaneously performed. .

The composition of the ladle slag during and after slag reforming is such that the basicity CaO / SiO ₂ is 5 ±.
The range is 0.2 and CaO / Al ₂ O ₃ is 1.
It is preferable to add burned lime to the tapped steel in advance so that the range is 5 ± 0.1.

The present invention has the following five features. First
At the same time, the slag reforming and the composition adjustment can be performed at the same time in the equipment that can be constructed at a low cost and has only the molten steel stirring and alloy feeding functions. Being able to improve productivity, third
In addition, by performing slag reforming after RH decarburization refining, the RH decarburization treatment can be efficiently performed without being affected by the decrease in [O] concentration in the molten steel.
Since slag modification is performed by stirring 1 and slag,
It is possible to improve the reaction efficiency and the uniformity of slag reforming. Fifthly, since the burnt lime is charged in the converter in advance so that the slag is melted at the molten steel temperature during the slag reforming treatment, the plasma That is, it does not require heating equipment for melting slag such as arc and arc.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings. As shown in FIG. 1, a ladle 4 is used in a steelmaking process as a molten steel storage container of a simple refining equipment. Molten steel 2 and slag 3 are housed in the ladle 4. The injection port 10a of the gas injection lance 10 is immersed in the molten steel 2 in the ladle so that the molten steel 2 and the slag 3 are gas-stirred. Argon gas and nitrogen gas are supplied to the gas injection lance 10 from a gas supply source (not shown) equipped with a flow rate control valve through a pipe 11.

The measuring portion 12 of the sublance 13 is immersed in the molten steel 2 and the slag 3. Sublance 1
The measuring unit 12 of No. 3 is used for measuring the temperature of molten steel and slag, measuring the oxygen concentration in molten steel, and sampling. The lances 10 and 13 are supported by an elevating device (not shown) so that they can be moved up and down.

In this embodiment, the ladle 4 is covered with the upper lid 5 to maintain airtightness to some extent. The gas blowing lance 10 and the sub lance 13 are inserted into the ladle 4 through the insertion hole of the upper lid 5. The reduction reaction of oxides (oxides such as iron and manganese) in the slag in the ladle is promoted by the heat supplied from the molten steel and the agitation of aluminum and gas in the molten steel. In addition, the cleanliness of molten steel is improved by the agglomeration of inclusions by gas agitation and the effect of promoting floating.

Further, a plurality of hoppers 7 are provided above the ladle 4, and predetermined amounts of auxiliary materials such as alloy and flux weighed by the weighing machine 8 pass through the shooters 9 from the hopper 7 into the ladle 4. It is supposed to be thrown in.

Next, with reference to FIGS. 2 and 3, Tables 1 and 2, the method of the embodiment will be described in comparison with the method of the comparative example in terms of productivity, slag composition and slag reforming. 2 shows a time chart when refining molten steel using the method according to the embodiment of the present invention, and FIG. 3 shows a time chart when refining molten steel using the method of the comparative example (conventional method). .

First, the two are compared in terms of productivity. The present inventors have found that the converter steelmaking time is 30 minutes (= T ₁ ) and the RH degassing refining time is 40 minutes (= T ₂ ).
Minutes, 10 minutes for deoxidation and component adjustment, 12 for treatment preparation time
In the refining process of continuous casting of ultra-low carbon steel consisting of (1)), RH degassing refining performs only vacuum decarburization and molten steel deoxidation by inputting aluminum, and ladle operation when component adjustment is performed in another facility Was simulated. The result is shown in FIG. As is clear from the figure, the lead time corresponding to the time from tapping to casting in the first heat to the fifth heat in the CC casting facility was 85 minutes.

Here, the breakdown of the time required for transfer between processes is about 20 minutes from the converter to the RH degassing equipment, and R
About 5 minutes from H degassing equipment to simple ladle refining equipment, RH
It takes about 20 minutes from the degassing equipment to the CC casting equipment and about 20 minutes from the simple ladle refining equipment to the CC casting equipment. When the converter steelmaking time is T ₁ , the RH degassing and refining time is T ₂ , and the simple ladle refining time is T ₃ , respectively, the converter steelmaking time T ₁ and the refining process pitch times T ₂ and T ₃ thereafter ... And have the relationship of the following formula (1).

T ₁ ≧ T ₂ , T ₃ (1) On the other hand, in the conventional method, as is clear from FIG.
The converter steelmaking time T ₁ is the same as 30 minutes and the RH degassing refining time T ₂ is 40 minutes, but the lead times for each heat in the CC casting equipment are 120 minutes (first heat) and 110 minutes (first heat), respectively. 2 heats), 100 minutes (3rd heat), 90 minutes (4th heat), and 80 minutes (5th heat).

As a result, it has been found that the method of this embodiment significantly reduces the average lead time per heat as compared with the conventional method. In this simulation, the RH decarburization time is expected to be shortened to about 3 minutes by changing the slag reforming position after the RH degassing treatment so that the RH decarburization refining is not affected by the slag reforming. . As a result, in the RH degassing treatment of ultra-low carbon steel, only vacuum decarburization treatment and deoxidation with aluminum etc. were performed, and the composition adjustment was carried out in a gas bubbling facility equipped with a molten steel stirring function and a ferroalloy charging function. .

The reason why the vacuum decarburization process is performed in the RH degassing facility is that the vacuum process is indispensable.
The reason for performing deoxidation treatment of molten steel in the RH degassing facility is that deoxidation as quickly as possible causes a smaller temperature drop of molten steel and less melting loss of refractory. Furthermore, the reason why the components are adjusted in the gas bubbling facility is that the ultra-low carbon steel does not require further vacuum treatment as long as it has a stirring function for the components adjustment. The above-mentioned gas bubbling equipment is usually installed immediately before the CC casting equipment in order to equalize the molten steel composition and the temperature before continuous casting, and it goes without saying that it does not hinder logistics. It is also possible to install it at low cost by modifying the existing one.

Next, the composition of the slag will be described. One of the important roles of ladle slag is the absorption of inclusions in molten steel, but this requires that the slag be in a molten state and highly fluid. On the other hand, in order to reduce the concentration of harmful oxides (oxides of iron and manganese) in the slag and reduce its activity, the basicity of the slag (CaO / SiO 2
₂ ) Need to be higher. However, if the basicity of the slag becomes too high, the slag approaches the physical properties of lime, so the melting point of the slag rises and the fluidity of the slag is impaired,
The Al ₂ O ₃ absorption capacity of the slag deteriorates.

From the above, in order to bring the slag into a molten state only by the heat retained by the molten steel without heating the slag by plasma or arc, etc.
It is also necessary to consider other oxide compositions in slag. In this example, the molten steel temperature after the RH degassing treatment was 1
To make the slag molten under the condition of 600 ° C,
CaO-SiO ₂ -Al basicity CaO / SiO ₂ from ternary phase diagram of the ₂ O ₃ and approximately 5, and CaO / Al ₂ O
The slag composition during and after slag reforming was defined as ₃ to about 1.5.

Next, the reducing agent used for slag reforming and its supply method will be described. The present invention is limited to the method for producing an aluminum-killed ultra low carbon steel, and Al is used as a reducing agent from the viewpoint of preventing [C] contamination of molten steel. However, during the slag reforming process, A
When 1 is added, the added Al floats on the slag and is oxidized and consumed by contact with the atmosphere, resulting in a decrease in reaction efficiency. Therefore, in the method of the present invention, Al in molten steel is used as much as possible to modify the slag by the interfacial reaction between the slag and the molten steel.

In this embodiment, the amount of Al contained in the molten steel as a reducing agent must be such that the oxidizing slag can be sufficiently modified and the final target [Al] is satisfied. . Although the amount of Al required for this slag reforming mainly depends on the slag composition and the amount of slag, it can be roughly estimated accurately by analyzing the slag composition at the initial stage of the RH degassing treatment as in this example. .

In the present embodiment, the burnt lime is introduced into the steel output from the converter, which utilizes the strong stirring power of the steel output from the converter and the large thermal energy of the high temperature molten steel in the steel output from the converter. hand,
This is for facilitating the melting of the added lime as much as possible.

Next, the method of the embodiment will be described in comparison with the method of the comparative example with reference to Tables 1 and 2. Five consecutive castings having the same steel type as the composition shown in Table 2 were tapped from the converter, and a comparative test was performed to compare the method of this example with a conventional method as a comparative example.

The manufacturing process of this embodiment includes a converter refining process,
A step of adding burned lime (1000 kg / heat) during tapping of the converter, a RH degassing step (vacuum decarburizing treatment with molten steel in a non-deoxidized state, and a subsequent deoxidizing treatment by adding Al);
It consists of a simple ladle refining process (mainly adjusting Mn and Al components and stirring slag and molten steel) and a continuous casting process.

The manufacturing process of the comparative example includes a converter refining process, a process of adding burned lime when the converter is tapped (1000 kg / heat), and Al added on the slag after tapping is completed (200 kg /
Heating), RH degassing (vacuum decarburizing with molten steel in a non-deoxidized state, followed by deoxidation by adding Al and mainly adjusting Mn and Al components), and continuous casting step Consists of.

Table 1 shows the details of the test contents of the example method and the comparative example method. As is clear from Table 1, the method of the present example has the following 5 compared with the method of the comparative example (conventional method).
It turned out to have one effect.

First, since the gas bubbling reacts the slag with the molten steel containing aluminum which is a deoxidizing element, the amount of T.Fe in the slag after the treatment is lower than that of the comparative example, and the oxidation degree of the slag is low. Of the molten steel sample in the tundish (T.
O) As can be seen from the analysis value, the cleanliness of the molten steel was significantly improved.

Secondly, the amount of (TO) before the RH treatment can be maintained at a high level, so that the vacuum decarburization rate is high and the vacuum decarburization treatment time can be shortened. Third, the RH vacuum processing time was shortened and the steam consumption rate was reduced.

Fourthly, the lead time was short, and the productivity of the refining process could be improved. Fifthly, as a result of shortening the process lead time, it is possible to reduce the steel tapping temperature of the converter, and as a result, the life of the furnace body is extended, the converter solvent solvent unit consumption is reduced, and the total output Steel yield improved.

In addition, since all heats can be manufactured with the same lead time, refining in the same process for all heats (uniform refining process) becomes possible, and stability of operation of converter and RH degassing process It was found that there is a possibility of improvement and the accuracy rate of the component quality.

Further, by performing the full heat RH degassing process, the operating rate of the RH degassing equipment is improved and the refractory of the RH degassing furnace is always kept at a high temperature, so that the life of the refractory can be extended. It turned out that there is a property.

According to the above-mentioned embodiment, the slag floating in the ladle is collected at the initial stage of the RH degassing process to analyze the composition, and the thickness of the slag floating in the ladle is measured.
Since the slag weight in the ladle is calculated from this measured value and the molten steel stirring time for slag reforming and molten steel component adjustment, and the addition amount of Al are determined from these analytical values and calculated values, It is possible to provide a manufacturing method with high Al efficiency.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

The effects brought about by the present invention are listed below. (1) It is possible to significantly improve the productivity of a steel factory. (2) By using gas bubbling for the component adjustment treatment, the degree of oxidation of slag is reduced and the cleanliness of molten steel can be improved. (3) The unit consumption of steam in the RH degassing process can be reduced. (4) It is possible to reduce the converter tapping temperature, extend the life of the converter refractory, reduce the converter solvent solvent unit consumption, and improve the total tapping yield. (5) The refining process can be standardized, and the accuracy of the ingredients and quality is improved. (6) As a result of enabling RH degassing treatment for all heat, the refractory of the RH degassing furnace is always kept at a high temperature,
The refractory life is extended.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a refining equipment used in a method for refining molten steel according to an embodiment of the present invention.

FIG. 2 is a time chart of a molten steel refining method according to an embodiment of the present invention.

FIG. 3 is a time chart according to a conventional method.

[Explanation of symbols]

 4 ... Ladle, 5 ... Upper lid 7 ... Hopper 10 ... Gas injection lance, 13 ... Sublance,

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication C21C 7/10 C21C 7/10 A (72) Inventor Gouji Toyota 1-chome, Marunouchi, Chiyoda-ku, Tokyo No. 2 Nihon Steel Pipe Co.

Claims (2)

[Claims] 1. A method for refining molten steel, in which molten steel roughly decarburized in a refining furnace is tapped in a ladle, and then the molten steel in the ladle is vacuum decarburized in an RH degassing facility. Addition to lower the melting point of slag, then vacuum decarburize the undeoxidized molten steel with RH degassing equipment, and further perform aluminum deoxidation, and then separate the molten steel with a molten steel stirring function and an alloy addition function. The method for refining molten steel is characterized in that the slag and the molten steel are refined in the simple refining equipment so that the molten steel is sufficiently stirred, and the components of the molten steel are adjusted and the slag is reformed at the same time. 2. The composition of the ladle slag during and after slag reforming is such that the basicity CaO / SiO ₂ is 5 ± 0.2.
, And CaO / Al ₂ O ₃ is 1.5 ±
The method for refining molten steel according to claim 1, wherein burned lime is charged into the tapped steel in advance so as to be in the range of 0.1.